Intervertebral discs, located between the endplates of adjacent vertebrae, stabilize the spine, distribute forces between vertebrae and cushion vertebral bodies. A normal intervertebral disc includes a semi-gelatinous component, the nucleus pulposus, which is surrounded and confined by an outer, fibrous ring called the annulus fibrosus. In a healthy, undamaged spine, the annulus fibrosus prevents the nucleus pulposus from protruding outside the disc space.
Spinal discs may be displaced or damaged due to trauma, disease or aging. Disruption of the annulus fibrosus allows the nucleus pulposus to protrude into the vertebral canal, a condition commonly referred to as a herniated or ruptured disc. The extruded nucleus pulposus may press on the spinal nerve, which may result in nerve damage, pain, numbness, muscle weakness and paralysis. Intervertebral discs may also deteriorate due to the normal aging process or disease. As a disc dehydrates and hardens, the disc space height will be reduced leading to instability of the spine, decreased mobility and pain.
Sometimes the only relief from the symptoms of these conditions is a discectomy, or surgical removal of a portion or all of an intervertebral disc followed by fusions of the adjacent vertebrae. The removal of the damaged or unhealthy disc will allow the disc space to collapse. Collapse of the disc space can cause instability of the spine, abnormal joint mechanics, premature development of arthritis or nerve damage, in addition to severe pain. Prosthetic implants are often used to prevent collapse of the space. The implant must provide temporary support and allow bone ingrowth. Success of the discectomy and fusion procedure requires the development of a contiguous growth of bone to create a solid mass because the implant may not withstand the compressive loads on the spine for the life of the patient.
Many attempts to restore the intervertebral disc space after removal of the disc have relied on metal devices. U.S. Pat. No. 4,878,915 to Brantigan teaches a solid metal plug. U.S. Pat. Nos. 5,044,104; 5,026,373 and 4,961,740 to Ray; U.S. Pat. No. 5,015,247 to Michelson and U.S. Pat. No. 4,820,305 to Harms et al. teach hollow metal cage structures. There are several disadvantages associated with the use of these metal implants. Solid body metal implants do not allow bone ingrowth which may lead to the eventual failure of the implant. Surface porosity in such solid implants does not correct this problem because it will not allow sufficient ingrowth to provide a solid bone mass strong enough to withstand the loads of the spine. On the other hand, the hollow cage structures of Harms, Ray and Michelson allow ingrowth. These devices can also be filled with bone graft material to promote bone growth. Unfortunately, many of these devices are difficult to machine and therefore expensive. Furthermore, metal implants may stress shield the bone graft, increasing the time required for fusion to occur.
The Michelson implant further requires a special tool and additional preparation of the adjacent vertebral bodies to ensure fusion. A special press is required to forcibly inject a compressed core of osteogenic material into the device. The osteogenic material, which is removed from the patient's iliac crest, must be compressed so that the graft material extends through openings in the implant whereby the graft material directly contacts the bone of the adjacent vertebral bodies. Michelson also requires coring out an area of each adjacent vertebral body to provide sufficient surface area of contact between the implant and the cortical bone of the vertebrae.
The use of bone graft materials in these past metal cage fusion devices presents several disadvantages. Autografts, bone material surgically removed from the patient, are undesirable because they may not yield a sufficient quantity of graft material. The additional surgery to extract the autograft also increases the risk of infection and may reduce structural integrity at the donor site. The supply of allograft material, which is obtained from donors of the same species, is not limited. However, allografts are also disadvantageous because of the risk of disease transmission and immune reactions. Furthermore, allogenic bone does not have the osteogenic potential of autogenous bone and therefore will give only temporary support.
Due to the need for safer bone graft materials, bone graft substitutes, such as bioceramics have recently received considerable attention. Calcium phosphate ceramics are biocompatible and do not present the infectious or immunological concerns of allograft materials. Ceramics may be prepared in any quantity which is a great advantage over autograft bone graft material. Furthermore, bioceramics are osteoconductive, stimulating osteogenesis in boney sites, and are also thought to be osteogenic, able to initiate osteogenesis in non-boney sites. Bioceramics provide a porous matrix which further encourages new bone growth. Unfortunately, ceramic implants lack the strength to support high spinal loads and therefore require separate fixation before the fusion.
Of the calcium phosphate ceramics, hydroxyapatite and tricalcium phosphate ceramics have been most commonly used for bone grafting. Hydroxyapatite is chemically similar to inorganic bone substance and biocompatible with bone. However, it is slowly degraded. .beta.-tricalcium phosphate is rapidly degraded in vivo and is too weak to provide any support. These ceramics have proven unsatisfactory for providing temporary support after discectomy while awaiting fusion.
A need has remained for fusion spacers which stimulate bone ingrowth and avoid the disadvantages of metal implants yet provide sufficient strength to support the vertebral column until the adjacent vertebrae are fused.
A need has also remained for bone graft substitutes which provide the osteogenic potential and low risk of infectious or immunogenic complications of allograft without the disadvantages of autograft.